1. Field of the Invention
The present invention relates to a tapered carbon mecroelectrode and a process for producing said tapered carbon microelectrode.
More particularly, the present invention relates to a tapered carbon microelectrode for voltammetry, electrochemical detectors, probes and electrode for STM (Scanning Tunneling Microscope) and related method, sensors for environmental analysis, sensors for pathological test, probe electrodes for detection used for living body system, food and the like where harmlessness and innoxiousness are strictly required, and the like, and a process for the production thereof.
2. Description of Related Art
Electrochemical detection methods such as voltammetry, high performance liquid chromatography, sensors and the like are widely used for highly selective and highly sensitive measurements.
Therefore, the methods are often used for analysis and evaluation of clinical living body samples and environmental samples containing components to be measured only in extremely small amounts and many compounds coexisting therewith.
An important member determining the performance of the electrochemical detector is a working electrode, and the kind of the selected working electrode material largely influences on the kinds of substances which can be measured, sensitivity, selectivity, and possibility of quantitative analysis.
Heretofore, as the electrodes for electrochemical measurement, there have been investigated various mercury electrodes, platinum, gold, copper, silver, carbon (glasslike carbon) (GC), carbon fiber (CF), carbon paste (CP), highly oriented pyrolytic graphite (HOPG) and the like.
Among them, mercury electrode and GC are mainly used as an electrode for polarography and an electrode for an electrochemical detector, respectively.
However, mercury is not desirable from the standpoint of environmental protection due to its poisonous property, and thereby nonpoisonous and good electrodes in place of mercury electrode are in demand. Though a GC electrode is composed of nonpoisonous carbon, it is difficult to treat the surface of the electrode, reproducibility of measurement result is not good and further, its use has problems. Other metal electrodes are liable to poison a living body system and are expensive, and the electrochemical characteristics have both merits and demerits.
Some of the present inventors found that a composite carbon material GRC (graphite reinforcement carbon) which had been used as a mechanical pencil lead exhibited excellent characteristics as an electrode for electrochemical measurements and succeeded in using the lead as the electrode and proposed a graphite composite carbon electrode (GRC) which can be easily used (Japanese Patent Application No. Sho 63-78698) as the electrode.
Further, they succeeded in making the GRC thin (Japanese Patent Application No. Hei 1- 250772).
Recently it is very important to obtain physiological information at a local portion of a living body, for example, nerve cell system in vivo and in situ by means of an electrode for electrochemical measurement.
The above mentioned thin GRC has been also produced for such purpose. That is, it is demanded to measure the change with time of responses from nerve cells and secreted amounts of chemical transmitters from nerve cells by placing an electrode in the vicinity of the nerve cells in the living body to be measured or directly pricking an electrode into a living body to stimulate directly the nerve so as to give a physical, chemical or electrical stimulus.
Therefore, a very thin electrode nonpoisonous to cells (on the order of .mu.m) is necessary.
The thin GRC in one of the above-mentioned prior Japanese Patent Applications can be sufficiently used for this purpose and it has been already shown that a separate quantitative analysis of dopamine and vitamin C.
However, the GRC is anyhow so thin that the electric resistance is somewhat high and the mechanical strength is not sufficient though depending on the place to be pricked with the GRC and thereby a tougher thin electrode is demanded.
A research on fabricating an electrode by using a thin carbon fiber in a manner similar to above has been conducted, but it is difficult to find electrodes of good reproducibility, and further, the carbon fiber electrodes are not tough and the strength is almost similar to that of the thin GRC.
Probes having a thin tip portion have been recently used in STM which is a new field of application of electrode for electrochemical measurements and the related technical fields since electric current measurements and cyclic voltammetric measurements are conducted therein.
However, any materials now available are poor in reproducibility so that inexpensive electrodes for STM of good reproducibility are in demand.
STM measurements have been recently explosively developed and therefore, a microprobe and a microelectrode are very important since it influences the performance of a STM apparatus.
The advantage of using carbon microelectrodes as probes for STM is not only the property that carbon rod can be thinned, but also no electrochemical problem caused between different materials since a clean graphite plate (for example, HOPG) is often used as a substrate for bearing a sample to be measured and simultaneously functioning as a counter electrode and a tapered carbon microelectrode in which the atom of the probe portion, that is, the end portion of graphite crystal and amorphous carbon tip portion, is the same kind as that of the counter electrode-substrate. This advantage is also available in the present invention.
The present inventors have intended to solve the above-mentioned problems of electrode materials. A carbon microelectrode of Japanese Patent Application No. Hei 1- 250772 has the following features i) - vii).
i) A carbon microelectrode capable of applying any of electric current, voltage, and mechanical stimuli to a living body and the size of the functioning portion being of an order of cell. PA0 ii) A carbon microelectrode which does not contaminate the system to be measured and even if it remains in a living body, it is not harmful, and it can be used for examining food. PA0 iii) A carbon microelectrode having a mechanical strength sufficient to prick a living body and food for an electrochemical detection as to a microportion or microamount. PA0 iv) Little fluctuation of electrode characteristics, good reproducibility of data, and reliable measurement. PA0 v) Any special pretreatment is not necessary, and at most an electrochemical pretreatment is enough to measure stably an electrode reaction. PA0 vi) Low cost and disposable. PA0 vii) A carbon microelectrode having low electric resistance (good electric conductivity), high prick strength (toughness) and a very strong stem (hardly broken) as compared with carbon fiber electrodes (CF) and thin GRC electrodes. PA0 i) Wide polarized potential domain and small blank current. PA0 ii) Good reproducibility, and repeated use is possible. PA0 iii) Having an electrode reaction activity. PA0 iv) Individual electrodes do not have different characteristics from one another. PA0 v) Impurities are so little that electrode reaction is not disturbed. PA0 vi) Theoretical interpretation is possible. PA0 vii) Handling and pretreatment are easy. PA0 viii) Electric resistance is lowered. PA0 ix) Mechanical strength required for prick is elevated. PA0 x) The stem portion is strengthened. PA0 xi) The length and diameter of the microcylinder portion at the tip and the length of the taper portion can be optionally controlled. PA0 xii) A desired portion of the above-mentioned electrode having such irregular shape can be sufficiently insulated. PA0 (i) extruding an organic material itself or a composition prepared by highly dispersing and compounding crystalline carbon fine powder with an organic material as a binder into a desired thin rod form, PA0 (ii) calcining the product thus extruded in an inert atmosphere or in a non-oxidizing atmosphere up to an elevated temperature, PA0 (iii) thereby carbonizing the organic material itself or the organic binder contained in the composition to produce a pure carbon thin rod, PA0 (iv) soaking the resulting carbon thin rod as an anode in an electrolytic solution, at near the oxygen evolution potential, PA0 (v) gradually pulling up the carbon thin rod while subjecting the tip portion of the carbon thin rod to electrochemical oxidation to form a conically sharp tip portion having an extremely small diameter, PA0 (vi) then connecting a lead wire with the thick end portion of the carbon thin rod, and PA0 (vii) coating with an insulating material all the surface of the carbon thin rod except a desired portion at the conically sharp tip portion having an extremely small diameter. PA0 (i) extruding an organic material itself or a composition prepared by highly dispersing and compounding crystalline carbon fine powder with an organic material as a binder into a desired thin rod form, PA0 (ii) calcining the product thus extruded in an inert atmosphere or in a non-oxidizing atmosphere up to an elevated temperature, PA0 (iii) thereby carbonizing the organic material itself or the organic binder contained in the composition to produce a pure carbon thin rod, PA0 (iv) soaking the resulting carbon thin rod as an anode in an electrolytic solution, at near the oxygen evolution potential. PA0 (v) gradually pulling up the carbon thin rod while subjecting the tip portion of the carbon thin rod to electrochemical oxidation to form a conically sharp tip portion having an extremely small diameter, PA0 (vi) then connecting a lead wire with the end portion of the carbon thin rod, and PA0 (vii) coating with an insulating material all the surface of the carbon thin rod except a desired portion at the conically sharp tip portion having an extremely small diameter.
The present inventors have intended to produce a carbon microelectrode having the above-mentioned features i) - vii) in addition to solving the above-mentioned problems of electrode materials.
In general, essential conditions for a voltammetric electrode are as shown below.
According to prior filed Japanese patent applications, that is, Japanese Patent Application Nos. Sho 63-78698, Hei 1-250772 and Hei 2-1051, carbon microelectrodes are disclosed which can be sufficiently used directly. However, in the case of using for measuring potential only, they have undesirably high electric resistance and furthermore, the mechanical strength is not enough to prick a somewhat hard matter so as to measure and apply an electric stimulus. The shape of those electrodes are not appropriate when they are used as an STM probe electrode which desirably has a shape such that the tip portion only is to be very thin. In addition, there has been demanded an electrode having a thick and strong upper portion while only the tip portion has a thinness enough for the purpose of electrochemical measurement.
If the above-mentioned problems can be solved, the following conditions also can be satisfied.
The present inventors have intensively researched so as to attain the characteristics viii) to xii) as well as i) to vii), in addition to solving the above-mentioned problems and producing a carbon microelectrode having the previously mentioned features i) - vii). As a result, the present invention has been completed.